1. Field of the Invention
The present invention relates to a process for forming a thin film of TiSiN, in particular for phase change memory devices.
2. Description of the Related Art
As is known, phase change memory devices utilize a class of materials that have the unique property of being reversibly switchable from one phase to another with measurable distinct electrical properties associated with each phase. For example, these materials may change between an amorphous disordered phase and a crystalline, or polycrystalline, ordered phase. A material property that may change and provide a signature for each phase is the material resistivity, which is considerably different in the two states and varies of two or more orders of magnitude when the material transforms from the amorphous phase (more resistive) to the crystalline phase (more conductive) and vice versa.
Phase change may be obtained by locally increasing the temperature. Under 150° C., both phases are stable. Over 200° C., nucleation of crystallites is fast and if the material is kept to the crystallization temperature for a sufficient time, it changes phase and becomes crystalline. In order to change the phase back to the amorphous state, its temperature is brought over the melting point (about 600° C.) and rapidly reduced.
From an electrical point of view; it is possible to reach both critical temperatures (crystallization and melting temperatures) by passing an electric current through a resistive electrode in contact or close proximity with the chalcogenic material and heating the material by Joule effect.
A chalcogenic element 1 based on the above is shown in FIG. 1, and comprises a resistive electrode 2 and a chalcogenic region 3. The chalcogenic region 3 is generally in the crystalline state to allow good current flow. A portion of the chalcogenic region 3 is in direct contact with the resistive electrode 2 and forms a phase change portion 4.
By passing an electrical current of suitable value through the resistive electrode 2, it is possible to selectively heat the phase change portion 4 to the crystallization or melting temperatures and cause a phase change.
The present requirements of low-energy devices impose some constraints to the program current of the device. In order to meet such requirements, the resistive electrode should comprise a film with the following features:
a) very low thickness (5-50 nm);
b) high conformity, to ensure the electrical continuity of the electrode;
c) medium resistivity (i.e., 0.5-5 mΩ·cm);
d) temperature stability.
The presently available processes are not able to ensure all the above features. Indeed, deposited films with similar characteristics to the above are deposited using a PVD (Physical Vapor Deposition) or a CVD (Chemical Vapor Deposition) technique.
However, films deposited by PVD are scarcely conformal due to the specific deposition technique and thus cannot ensure the electrical continuity.
Currently available CVD processes, in particular CVD processes developed to deposit TiSiN layers, are all optimized to form low-resistivity films, in particular to form barrier layers in interconnects, and thus are not compatible with the requirements.
For example, C. Marcadal, M. Eizenberg, A. Yoon, L. Chen “Metallorganic Chemical Vapor Deposited TiN Barrier Enhancement with SiH4 Treatment” in Journal of The Electrochemical Society, 149 (1) C52-C58 (2002) discloses a commercial process using Metallorganic Chemical Vapor Deposition (MOCVD). In detail, this known process comprises three basic steps:
1. depositing a TiN film at medium temperature (350-450° C.) by thermal decomposition of a metallorganic precursor, namely TDMAT (Tetrakis Dimethylamino Titanium);
2. exposing the TiN film to H2/N2 plasma;
3. exposing the TiN film to a silane (SiH4) flow for 10 s.
All the steps are carried out in the same reaction chamber.
The film obtained from the deposition in step 1 is very rich in carbon (30% atomic); carbon, present in the metallorganic precursor, causes the deposited film to be unstable when exposed to air (the film oxides and its resistivity increases in an uncontrollable way). The plasma treatment reduces the carbon content and at the same time thins the layer, so that the latter is less permeable to O2 (a thickness reduction is indeed observed following the plasma treatment). The subsequent silane treatment causes addition of silicon to the film; however, in the final layer, the Si concentration is quite small (about 4.4 at%) since the reactive sites have been dramatically reduced by the plasma treatment. As used herein, at% refers to “atomic percentage”, i.e., the ratio of the number of a specific atomic component in a composition relative to the total number of all the atoms of the composition.
Since in the literature the resistivity of the end film is attributed to the presence of Si—N covalent bonds, it is clear that the known process can yield only a low resistivity film, since here the Ti—N metal bond is predominating.
Another known process (see, e.g., “Low Resistance Copper Interconnects with MOCVD TiN(Si) Barrier for Sub-0.13 μm Applications”, T. Suwwan de Felipe, et al., Novellus Systems and International SEMATECH) teaches the deposition of MOCVD TiNSi using TDEAT (Tetrakis Diethylamido Titanium) and ammonia and soaking in SiH4 in-situ for forming a barrier for copper. Also here, the process is studied so as to reduce the resistivity of the barrier and thus is not suitable for forming a resistive layer, in particular for use in PCM devices.